A typical organic light emitting device (OLED) comprises a glass substrate, a transparent substrate electrode, e.g. Indium-Tin-Oxide (ITO), an electroluminescent layer stack and a reflective counter electrode. Light generated from the device is emitted through the glass substrate. The indices of refraction of the substrate electrode, the electroluminescent layer stack and of glass are typically 1.9 to 2.0, 1.7 to 1.9, and about 1.5, respectively.
Due to the differences between the refractive indices of the substrate, the layers and the environment, usually air, most of the light generated in the OLED is not able to escape from the substrate to the outside. Typically, the generated light is divided into the following fractions: 30% of the light is trapped in the light guide formed by the organic layers and the ITO, about 20% is lost in the interaction with the metal-cathode, about 50% enters the glass substrate and from the glass substrate only 20% can escape to the outside. According to the prior art, different means for an improved light outcoupling are already known, e.g. layers with light scattering properties, but unfortunately leading to different disadvantages.
As an example, EP 1 734 792 A1 discloses an electroluminescent device comprising a reflective electrode with light-scattering and light-reflecting properties. Unfortunately, said structuring of the substrate electrode is limited within the nm-range and is limited to only a low improvement of said light outcoupling.
EP 1 763 081 A2 discloses an OLED with a substrate featuring a number of recesses, in which a non-transparent reflective material is buried in, in order to reflect light captured within the substrate to the substrate surface with another propagation direction to couple out the light to the environment. Unfortunately, the manufacturing of a substrate comprising recesses in order to embed in said non-transparent reflective material is complex and costly. Moreover, the amount of emitted light, captured within the layers on top of the substrate will not be coupled out with this measure.